Questions related to the perception and measurement of light (primarily in the visible range), its mathematical description, the reproduction of colors by different means, color combinations, etc. Please use the tag [electromagnetic-radiation] if you want to refer to the general form of light.
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3answers
410 views
Why do prescription glasses that darken in sunlight fail to darken when the light travels through a car windshield?
I was going to buy this add on, as I almost exclusively use my sunglasses for driving. I'm just stacking them over my normal ones now... but it's quite annoying that I almost paid the extra $$$ for ...
3
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4answers
231 views
Eye sensitivity & Danger signal
Why are danger signal in red, when the eye is most sensitive to yellow-green?
You can check luminosity function for more details...
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3answers
210 views
Why frequency doesn't change during refraction?
When light goes through one medium to another it's velocity and wavelength changes. Why frequency doesn't change in this phenomenon?
3
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4answers
143 views
Rainbow around Sun
From the perspective of a person, a rainbow is formed when the Sun is behind the person, and there is a critical angle made by the rainbow.
However, on several occasions, usually at noon when the Sun ...
3
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2answers
162 views
Why do hot objects prefer to emit photons over electrons ? Is there electron-positron annihilation?
Why do hot objects prefer to emit photons over electrons ? Is there electron-positron annihilation ? If so , why ? Im confused by this.
3
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1answer
794 views
Purple doesn't occur in rainbow - or does it?
Usually, when asked whether the purple color exists rainbows, an answer similar to this is given:
The purple color is perceived by human eyes via the activation of both red-sensitive and ...
3
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2answers
41 views
Color Variation in RR Lyrae
I've been doing some research on RR Lyrae stars and haven't been really able to find an answer to this question.
RR Lyrae are well known for their periodic magnitude, and also are usually found in ...
3
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1answer
31 views
Assuming an observer is 50 light years away, in the plane of the solar system and observing earth, what is the light flux of earth he would see?
As in, from what the planet emits and re-radiates out into space (this is going to be important if we are to image any Earth-like planet)
3
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1answer
88 views
What the name of the evacuated glass gadget with black and white vanes that turn when a light is applied?
I remember a glass device my physics teacher had at high school which
Contained some vanes mounted somehow on a vertical axis, which were all black on one side and white on the other
Was in a ...
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2answers
299 views
Why do electrons around nucleus radiate light according to classical physics
As I navigate through physics stackexchange, I noticed Electron model under Maxwell's theory.
Electrons radiate light when revolving around nucleus? Why is it so obvious?
Note that I do not know ...
3
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3answers
170 views
Why does it require such little energy to create the fastest thing in the universe?
I have noticed when I turn on the light switch in my house light comes from the bulb.
How is this light created?(process occurring in the bulb) and why is this small amount of electricity enough to ...
3
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2answers
530 views
Propagation of light in transparent media: absorption and reemission or scattering?
In the two Phys.SE questions What is the mechanism behind the slowdown of light/photons in a transparent medium? and Why glass is transparent? transparent media were discussed. But I'd like to clarify ...
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10answers
584 views
Why do we think of light as a wave?
I've read that light travels in a straight line and has a wavelength of 400nm to 700nm. But I don't understand why does it have a wavelength and what creates its wavelength? I agree with the concept ...
3
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1answer
31 views
Looking for a resource that explains all colors and shapes of stars and galaxies
I've been looking at some of the astronomy pictures of the day from NASA. They all have content that is indicative of certain situations or events. Supernovas have the scattered lines look, new stars ...
3
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3answers
777 views
Conservation of energy and Doppler effect?
From what I understand, the frequency of light coming from the source moving towards an observer increases. From $ E=hv $ , this implies increase in energy of photon.
What really is confusing, is ...
3
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1answer
283 views
simple test/measurement to quantify water opacity
I am performing some measurements on liquid surfaces and I want to quantify how these mesures depend on the liquid opacity by performing it with different concentrations of colorant. To allow ...
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2answers
2k views
What invisible flicker do different types of light bulbs have?
What waveform does the light coming off different types of artificial light sources have and (of course) what are the underlying physical reason for them?
I was inspired to ask this question ...
3
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1answer
233 views
What are the coloured bands in this “Pale Blue Dot” picture?
You can see Earth in the right most band in this image:
What are these bands doing in space?! Can somebody please explain the coloured bands in this image?
3
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1answer
202 views
Lunar twilight and sixth magnitude stars
Summary: when the Moon is x degrees below the horizon, it interferes
with stargazing the same as astronomical twilight would. What is x (as a
function of the Moon's phase)?
We define civil, ...
3
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2answers
72 views
Is light's path a wave?
In a lot of textbooks I see a schematic of light drawn as a squiggly line. I have even heard that some things are too small to be seen because they are smaller than the wavelength of light (and ...
3
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1answer
115 views
Does light induce an electric current in a conductor?
I know that electromagnetic waves induce electric currents in conductors and that's the basis for radio, wi-fi etc.
I also know that light is also an electromagnetic wave. So, can light induce a ...
3
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1answer
87 views
Angle of rays leaving a light source
I'm working on ray tracing and I'm trying to understand the impact of the angle at which a light-path intersects the surface of a light source on the amount of light that source delivers to the path ...
3
votes
1answer
38 views
How do you transform between theoretical Hertzsprung-Russell Diagrams and Colour-Magnitude Diagrams?
When discussing stars, theorists tend to use effective temperature $T_\text{eff}$ and luminosity $L$ (on logarithmic scales). Observers, on the other hand, usually talk about observed colours and ...
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2answers
680 views
Can 2 beams of ultraviolet light intersect and be visible where they intersect?
Is it possible that if you have 2 ultraviolet lasers, that are invisible to the human eye, and if you aim their beams to intersect at some point, that the place of intersection will show a lower ...
3
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1answer
72 views
Can you “fold” EM or light waves? (i.e) long wave that is reflected by mirror in fragments - like in the game “Snake”
So, I was reading about the Casimir effect. Two mirrors facing each other attract to each other in a vacuum. The reason is due to pressure exerted on those mirrors from the multitude of EM waves (like ...
3
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1answer
78 views
How would I perceive a purple ball when traveling at relativistic speeds
I have a 'thought experiment' which I'd like to ask about.
I was thinking what speed I would have to drive towards a crossing to see a red light as a green light - pretty easy, using the doppler ...
3
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1answer
121 views
Was the choice of red/green colors for traffic lights a happy coincidence?
All of us have learned since secondary school that the reason we use red in the traffic lights for "Stop" is because it travels the furthest without dispersion. According to wikipedia which cites ...
3
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1answer
271 views
Transparency of solids using bandgaps and relation to conduction and valence bands
I think I understand how a solid can appear transparent as long as the energy of the photons travelling through it are not absorbed in the material's bandgap. But how does this band gap relate to ...
3
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1answer
198 views
plotting an sRGB gamut Chromaticity Diagram starting with xy coordinates
Starting with xy coordinates I would like to plot a chromaticity diagram displaying just the sRGB gamut portion of the CIE xy chromaticity diagram.
For each ...
3
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3answers
331 views
formula for transparency of very thin film of metal
Is there formula that gives transparency of very thin film of given metal (tens of nanometers) to the visible light/light of given wavelength ? Which properties of metals are needed for the formula ?
...
3
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1answer
568 views
How do we see different colours?
Why do different wave lengths cause electrons to behave(?) differently, causing us to see different colors?
What is happening at the quantum level which causes the colour black to absorb all of the ...
3
votes
1answer
191 views
Is there a mathematical way to describe how a flame flickers?
I love the way candles flicker. It's a great effect and I almost want to see it replicated in an actual lightbulb. I was curious if there is any way to express that mathematically? I'm not that ...
3
votes
1answer
401 views
Formula for polarized “light” transmission through close filters?
I'm still trying to understand photons (or polarized electromagnetic radiation).
This question is similar to one of my previous questions, but different:
Consider the famous demonstration of crossed ...
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0answers
315 views
How to turn water opaque by pouring the smallest quantity of matter into it?
Consider a glass of water and a glass of coffee. Their contents differ by no more than a few grams of particles coming from the roasted and ground coffee, yet the former lets almost all visible light ...
3
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2answers
265 views
Why does light of high frequency appear violet?
When people are asked to match monchromatic violet light with an additive mix of basic colours, they (paradoxically) mix in red. In fact, the CIE 1931 color space chromaticity diagram shows this ...
2
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3answers
762 views
Is there something special in the visible part of electromagnetic spectrum?
I always wondered how much information we get from color. Things we see have different colors; edible products change color when began to spoil so we have a notion what color a fresh product should ...
2
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4answers
268 views
How are we able to view an object in a room with bulb..?
This is a very basic question on optics. How are we able to view an object kept in a room with a bulb?
From what I understand, light rays from bulb will hit the object and some colour will be ...
2
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4answers
464 views
Why are color values stored as Red, Green, Blue?
I learned in elementary school that you could get green by mixing blue with yellow.
...
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3answers
7k views
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4answers
1k views
Is it only red, green and blue that can make up any color through additive mixture?
I'm reading about color vision and have some trouble understanding the motivation for why the trichromatic theory was suggested in the first place. The book I'm reading ("Psychgology: The science of ...
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1answer
1k views
How long does it take for radio or light waves to travel from Earth to Jupiter, Saturn, Uranus, Neptune and Pluto?
I know it roughly takes 20 minutes from Earth to Mars, and 8 minutes from the Sun to Earth, but don't know how long for the other planets on my list.
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3answers
971 views
Newton's corpuscular theory
Where did Newton get the idea that light had a particle nature and not a wave nature? At those times, AFAICT there were no phenomena that showed particle nature. But wave nature is much easier to ...
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2answers
665 views
A light flickers at a fast rate why do humans only see light on?
A light switches on and off at a fast rate, with equal time off and on, why do we see a light that appears only on rather than only off?
2
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2answers
943 views
If blue light has a higher energy than red light, why does it scatter more?
As $E=hf=\frac{hc}{\lambda}$, blue light - with a smaller wavelength - should have a higher energy. However, it is the case that blue light scatters the most. Why is it that higher energy rays scatter ...
2
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1answer
149 views
If we were to travel through space (sci-fi style), how close to the false-color images would the galaxies we see be?
I understand that the black-and-white images you see looking through a household telescope are only like that due to the intensity of the light that reaches us, and that most of the astronomy images ...
2
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1answer
249 views
how many times have the first photons lapped the universe?
I'm not really interested in the actual number of times, but rather an explanation of a closed Universe. How do we still measure the background radiation of the Big Bang? Those electromagnetic waves ...
2
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1answer
318 views
Snell's Law of Refraction
I was told that "Snell's law of refraction implies that a light ray in an isotropic medium travels from point a to point b in stationary time." Why is this true?
Thanks.
2
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2answers
694 views
Why can you see a light in the location that the light source just left?
This is my first question on this site so forgive me for the awkward wording of the question. Basically, my question is why does light from, say, a sparkler, seem to remain where it just came from to ...
2
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1answer
114 views
Can light be canceled by merging with an inverted wave?
Can light waves be canceled by merging them with their inverted waves? Seems like it would violate conservation of energy but waves are added together when they overlap, right? Where is the flaw in ...
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3answers
271 views
What was the motivation behind suggesting the trichromatic theory of vision?
Background
In this thread, I asked whether it is true that the colors red, green and blue, through additive mixture, can make up any color. Turns out they can't.
However, when reading about the ...
